Inhalation of high concentrations of low toxicity dusts in rats results in impaired pulmonary clearance mechanisms and persistent inflammation.

This study was carried out to assess the time course of pulmonary clearance impairment and persistence of inflammation following high-dose inhalation exposures to titanium dioxide (TiO2) or carbonyl iron (CI) particles. Male rats were exposed to air, TiO2 or CI particles 6 hr/day, 5 days/week, for 4 weeks at concentrations of 5, 50, and 250 mg/m3 and evaluated at selected intervals through 6 months postexposure. Indices of pulmonary inflammation as well as alveolar macrophage clearance functions (i.e., morphology, in vivo and in vitro phagocytosis, and chemotaxis), cell proliferation, and histopathology endpoints were measured at several postexposure time periods through 6 months. In addition, amounts of TiO2 or CI in lungs and tracheobronchial lymph nodes were measured to allow an evaluation of particle clearance and translocation patterns. Four-week exposures to TiO2 or CI particles at concentrations of 250 mg/m3 resulted in lung burdens of 12 mg titanium and 17 mg iron, respectively, with particle retention half-times ranging from 68 days for 5 mg/m3 TiO2 to approximately 330 days for 250 mg/m3. The impact of this TiO2 dust load and similar lung burdens of CI particles produced a sustained pulmonary inflammatory response measured through a period of 3-6 months postexposure concomitant with increases in BrdU cell labeling of terminal airway and pulmonary parenchymal cells. The impairment of particle clearance mechanisms was accounted for by deficits in in vitro phagocytic and chemotactic potential of alveolar macrophages recovered from the lungs of high-dose, TiO2- or CI-exposed rats. Free granular pigment (TiO2 or CI) was present on the hypertrophic mucosal surfaces of bronchioles and bronchi, and particle-laden macrophages, found individually, were numerous throughout alveoli and within lymphoid tissues immediately after exposure. Aggregates of particle-laden macrophages were present within alveoli and alveolar ducts from 1 week postexposure through the entire 6-month recovery period. Macrophage accumulations increased in size and number from 1 week through 1 month postexposure and then appeared to remain constant through the remaining 5-month postexposure period. Minimal cellular hypertrophy and hyperplasia were evident at alveolar duct bifurcations adjacent to macrophage aggregates, and this effect was most prominent at 3 to 6 months postexposure. The results of this study clearly demonstrate that exposure to high dust concentrations of two different innocuous particle types produced sustained pulmonary inflammation, enhanced proliferation of pulmonary cells, impairment of particle clearance, deficits in macrophage function, and the appearance of macrophage aggregates at sites of particle deposition. In addition, the mass deposition rate determination appears to be a less sensitive indicator of "overload" when compared to biomarkers of pulmonary toxicity, such as macrophage function and cellular inflammation and proliferation indices.